A mobile terminal such as a mobile telephone or a mobile game machine uses a liquid crystal display device as its displaying means, in general. In addition, since the mobile telephone is driven by a battery, it is strongly required to reduce power consumption. Therefore, information such as a time or remaining battery level which needs to be constantly displayed is displayed on a reflective subpanel. In addition, recently, both normal display by way of a full-color display and reflective constant display are required to be realized on the same main panel.
FIG. 39 shows an equivalent circuit of a pixel circuit of a general active matrix type liquid crystal display device. In addition, FIG. 40 shows a circuit arrangement example of the active matrix type liquid crystal display device having m×n pixels. In addition, each of the numbers m and n is two or more integer.
As shown in FIG. 40, a switch element composed of a thin film transistor (TFT) is provided at each intersecting point of m source lines SL1, SL2, . . . , SLm and n scanning lines GL1, GL2, . . . , GLn. In FIG. 39, the source lines SL1, SL2, . . . , SLm are represented by a source line SL, and similarly, the scanning lines GL1, GL2, . . . , GLn are represented by a scanning line GL.
As shown in FIG. 39, a liquid crystal capacitive element Clc and an auxiliary capacitive element Cs are connected in parallel through the TFT. The liquid crystal capacitive element Clc has a laminated structure in which a liquid crystal layer is provided between a pixel electrode 20 and an opposite electrode 80. The opposite electrode is also referred to as a common electrode.
In addition, in FIG. 40, as for the pixel circuit, the TFT and the pixel electrode (black rectangular part) are simply shown.
The auxiliary capacity Cs has one end (one electrode) connected to the pixel electrode 20, and the other end (the other electrode) connected to an auxiliary capacity line CSL, and is provided to stabilize a voltage of the pixel data held in the pixel electrode 20. The auxiliary capacity Cs has an effect of preventing the voltage of the pixel data held in the pixel electrode from fluctuating due to a leak current of the TFT, a fluctuation of electric capacity of the liquid crystal capacitive element Clc between a black display and a white display due to dielectric constant anisotropy of liquid crystal molecules, and a voltage fluctuation generated through parasitic capacity between the pixel electrode and a surrounding wiring. By sequentially controlling a voltage of the scanning line, the TFT connected to the scanning line is turned on, and a voltage of pixel data supplied to the source line is written in the corresponding pixel electrode with respect to each scanning line.
As for the normal display by way of the full-color display, even when display contents are still images, the same display contents are repeatedly written in the same pixel with respect to each frame. Thus, the voltage of the pixel data held in the pixel electrode is updated, so that the voltage fluctuation of the pixel data is minimized, and a high-quality display of the still image can be maintained.
Power consumption to drive the liquid crystal display device is mainly dominated by power consumption to drive a source line by a source driver, and roughly expressed by a relational expression shown in the following formula 1, wherein P represents power consumption, f represents a refreshing rate (the number of times to perform a refreshing action for one frame per unit time), C represents load capacity driven by the source driver, V represents a drive voltage of the source driver, n represents the number of scanning lines, and m represents the number of source lines. Here, the refreshing action means an action to apply the voltage to the pixel electrode through the source line while maintaining the display contents.P∝f·C·V2·n·m  (Formula 1)
Meanwhile, in the case of the constant display, since the display contents are still images, it is not always necessary to update the voltage of the pixel data with respect to each frame. Therefore, in order to further reduce the power consumption, a refreshing frequency is lowered at the time of this constant display. However, when the refreshing frequency is lowered, the pixel data voltage held in the pixel electrode fluctuates due to a leak current of the TFT. The voltage fluctuation leads to a fluctuation of display brightness (transmittance of liquid crystal) of each pixel, and this is recognized as a flicker. In addition, since an average potential is lowered in each frame period, a display quality could be lowered such that a sufficient contrast cannot be provided.
Here, as a method to solve the problem that the display quality is lowered due to the lowering of the refreshing frequency and to cut the power consumption at the same time in the constant display of the still image of the remaining battery level or the time display, a configuration is disclosed in the following patent document 1. According to the configuration disclosed in the patent document 1, a liquid crystal display can be implemented by both transmissive and reflective functions, and moreover, a memory part is provided in a pixel circuit in a pixel region in which the reflective liquid crystal display can be provided. This memory part holds information to be displayed in the reflective liquid crystal display part as a voltage signal. At the time of the reflective liquid crystal display, information corresponding to this voltage is displayed when the pixel circuit reads the voltage held in the memory part.
According to the patent document 1, since the memory part is composed of a SRAM, and the voltage signal is statically held, the refreshing action is not needed, so that the display quality can be maintained, and the power consumption is reduced at the same time.